Oligosaccharide moieties on glycoproteins participate in intercellular communication, cell differentiation, and proliferation, and cellular transformation is accompanied by changes in cell surface glycoproteins. However, little is known about the structural and functional relationship of the oligosaccharyl chains and the regulation of their expression. Among the sugars that comprise the carbohydrate side chains, the participation of sialic acid, present only as terminal sugars in glycoproteins, is perhaps the most extensively, albeit incompletely, studied. While at least five, and most likely more, sialyltransferases are required in the biosynthesis of the major sialic acid linkages found in mammalian glycoproteins, four have been purified to homogeneity and characterized enzymatically. There are no data on their primary sequences, gene structures, or genomic organization. Furthermore, regulation of expression of the individual transferases, and functional differences of the products of the individual transferase reactions remain completely unexplored. In the long range, this research is directed at the elucidation of the regulatory pathways that dictate the expression of the individual sialyltransferases and their catalytic end-products. Toward this goal, a detailed molecular analysis of the sialyltransferases is proposed. First, sialyltransferase mRNAs will be studied by the isolation of recombinant clones from cDNA libraries. Second, sialyltransferase genomic sequences will be isolated. Comparative analysis of genomic and cDNA sequences will be performed to gain an in-depth understanding of the sialyltransferase family. Third, using probes that can distinguish between different sialyltransferase gene products, the regulated expression of the sialyltransferases in differentiating F-9 cells and in tissues of whole animals will be examined.